In general, vehicles are equipped with a fuel system including a fuel tank, and evaporation gas of gasoline is produced in the fuel tank when gasoline fuel is used. The gasoline evaporation gas mainly includes hydrocarbon (HC), so when it is discharged without any treatment, it causes severe air pollution.
In order to prevent this problem, vehicles using gasoline fuel are equipped with a canister for collecting fuel evaporation gas. The canister has an adsorbent therein, so it adsorbs and keeps evaporation gas produced in a fuel tank and then sends it to a carburetor with external gas for combustion, thereby preventing evaporation gas from leaking outside.
However, to collect evaporation gas, a canister of the related art, as shown in FIG. 1, uses active carbon in a particulate powder type and has narrow and complicated vent channels, so gas cannot smoothly flow. Accordingly, large back pressure is generated and it increases the load applied to the entire device, so the adsorption/separation efficiency of evaporation gas is low.
In particular, legislation for enhancing regulation of evaporation gas from vehicles has recently been adopted in many countries. However, existing canisters cannot sufficiently control evaporation gas from vehicles and thus do not comply with new rules. Accordingly there is an urgent need to solve this problem.
One of the reasons that vehicles produce exhaust gas is that hydrocarbon remaining in active carbon porosities in a canister is exposed diurnal changes of temperature while a vehicle is parked for several days. Such a process is called “DBL (Diurnal Breathing Loss)”.
At present, the KOREAN Environment Agency regulates automotive DBL to under 1.2 g/day for vehicles sold from 2014 through revision of Clean Air Conservation Act, and a rule for regulating DBL to under 0.3 g/day from 2017 is currently under examination. Further, CARB (California Air Resources Board) in the U.S. has a plan to phase in rules for regulating not only automotive DBL to 0.3 g/day, but also to phase in regulation for DBL for a canister to under 20 mg/day from 2014 to 2022.
There are two methods for satisfying the DBL regulations. The first method is to decrease the cross-sectional area of the canister and increase the length of active carbon in a canister keeping evaporation gas or air, or to add an auxiliary canister having the structure described above to the air side of a canister. According to this method, it is possible to reduce hydrocarbon remaining in the active carbon by increasing the amount of air passing through the active carbon and reducing the speed of evaporation gas produced in a fuel tank and diffused to the atmosphere through an adsorbent in a canister in accordance with a diurnal change of temperature, so it is possible to reduce DBL. However, according to the canister structure described above, there is a limit in decreasing the cross-sectional area and increasing the length while satisfying conditions for evaporation gas or air flow required by an actual fuel system (see Korean Patent Application No. 2002-0089198, Korean Patent No. 0821732, and SAE Paper 2007-01-1090).
The second method, proposed in US 2007/0051345 A1 and Korean Patent Application No. 2010-0099075, is to provide a heater in a canister or at the atmosphere side to increase separation efficiency of evaporation gas remaining in active carbon. Since a heater is provided, evaporation gas remaining in active carbon is minimized and the separation efficiency of evaporation gas is increased by heating air flowing inside when a canister is attached/detached. However, according to this method, there is a need for an electric device or a complicated system for the heater and a safety problem may be caused by the heater.